Metallography: Principles and Practices - ASM International

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Metallography: Principles and Practice (#06785G) Author(s): George F. Vander Voort 44 METALLOGRAPHY Figure 1-35 Fractograph of wedge test specimen of cast iron. The white areas indicate the presence of iron carbide, while the dark areas indicate that flake graphite is present. Each test uses a different procedure to cast the sample. The chill test uses a small rectangular casting where one face is a chill plate, while the wedge test employs a wedge-shaped casting. In the wedge test, the cooling rate varies with the wedge thickness, and chill plates are not used in the wedge test mold. After casting a specimen with either method, the sample is broken and the fracture is examined (see Fig. 1-35). In the chill test, the depth of the chill (white iron) is measured. The appearance of the zone between the fully white and fully gray fractures plus the chill depth is an indicator of melting conditions. In the wedge test, the distance from the wedge tip to the end of the white fracture is measured, usually to the nearest Vyi of an inch. In the production of tool steels, the fracture test may be used to detect graphite in tool steels that intentionally incorporate graphite or in high-carbon tool steels that should be free of graphite. Figure 1-36 shows four fractured test discs of a high-carbon steel that contains considerable undesired graphite as a result of the accidental addition of aluminum. 1-5.4 Grain Size The prior-austenite grain size of high-carbon martensite steels, such as tool steels, can be assessed by comparing a fractured sample to a series of graded fractured specimens. The method is simple, accurate, and reproducible, and it is discussed in detail in Chap. 6. 1-5.5 Depth of Hardening Copyright © 1984 ASM International ® All rights reserved. www.asminternational.org The depth of hardening in case-hardened steels, such as carbon tool steels, can be assessed through the use of fractured samples. The P-F (penetration-fracture) test developed by Shepherd is commonly used for evaluating carbon steel heats [31].
Metallography: Principles and Practice (#06785G) Author(s): George F. Vander Voort Copyright © 1984 ASM International ® All rights reserved. www.asminternational.org MACROSTRUCTURE 45 Figure 1-36 Fractograph of fractured hardened macroetched discs of AISI Wl (1.3% carbon) tool steel that were excessively graphitized as the result of a high, undesired aluminum content. In this test, a 3 A-'m diameter, 4-in long sample is austenitized at the recommended temperature, quenched in brine, and fractured. The case depth is measured based on the change in fracture appearance. After fracturing, the surface is usually ground, etched, and its hardness tested to define the depth to a specific hardness. 1-5.6 Detection of Overheating Fracture tests have also been used to detect overheating during soaking prior to hot working. In this test, a rectangular section, roughly 1-in square by 3 to 4 in long, is cut from the suspect material. It is normalized, quenched and tempered to 321 to 341 HB (Brinell hardness number), and fractured at room temperature. The appearance of coarse-grained facets on the fracture indicates overheating [32]. 1-5.7 Evaluation of Quality Fractures of longitudinal or transverse sections cut from wrought products or castings have been used for many years to evaluate metal quality. The fracture can reveal texture, flaking, graphitization, slag, blowholes, pipe, inclusions, and segregation. The general fracture appearance can be classified as coarse or fine, woody, fibrous, ductile, or brittle. Fibrous fractures result from microstructural anisotropy produced by alloy segregation or inclusions. Longitudinal fractures in wrought iron exhibit a classic fibrous appearance. Woody fractures generally result from gross alloy or nonmetallic segregations.
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Metallography: Principles and Practices - ASM International

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